Inductors are passive electrical devices found in many integrated circuits, including radiofrequency integrated circuits (RFICs), multiple band passive matching networks, multiple band voltage control oscillator (VCO) tank circuits, and phase delay units. Inductors may be used singularly in an integrated circuit or arranged in pairs as differential inductors or transformers in the integrated circuit. In general, an inductor is a reactive element that can store energy in its magnetic field and tends to resist a change in the amount of current flowing through it. The performance of an inductor significantly affects the overall performance of the related integrated circuit and may even be a performance limiting component. On-chip or monolithic inductors are commonly fabricated on the same substrate as the remainder of the related integrated circuit. Inductors can be fabricated with a conventional metal-oxide-semiconductor (MOS) process or advanced Silicon Germanium (SiGe) processes.
Important parameters of on-chip inductors include inductance, Q (the quality factor), self-resonant frequency (inductance and capacitance values), and the chip area, all of which need to be optimized in the circuit design. The quality factor Q is a commonly accepted indicator of inductor performance in an integrated circuit and represents a measure of the relationship between energy loss and energy storage in an inductor. A high value for Q reflects a low substrate loss and a low series resistance.
On-chip inductors, which may take either a planar form (including line and planar spiral types) or a spiral form, may have either a fixed inductance or a variable inductance. Mixed signal and radio frequency applications commonly require variable reactive elements (e.g., inductors or capacitors) to achieve tuning, band switching, phase locked loop functions, etc. Such reactive elements are used in some type of circuit where the reactive element is resonated with another reactive element. The desired result is a resonant circuit that has a response that can be tuned from one frequency to another dynamically. One approach is to build the ability to switch an additional length of conductor into the signal line of an on-chip variable inductor into the circuit design. The additional length of conductor can be connected either serially or in parallel with the original length of conductor. Lengthening the signal line of the inductor alters its inductance value. However, conventional arrangements require some type of switch in the signal line of the variable inductor, which may deteriorate the Q value to an unacceptably low value for many mixed signal and radio frequency applications.
Consequently, improved constructions for an on-chip variable inductor are needed that overcome without these and other deficiencies of conventional variable inductors.